Subionospheric VLF perturbations associated with lightning discharges

Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 591 – 606

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Subionospheric VLF perturbations associated with lightning discharges Craig J. Rodger∗ Department of Physics, University of Otago, PO Box 54, Dunedin, New Zealand

Abstract The use of very-low frequency (VLF) transmissions propagating inside the waveguide formed by the Earth and the lower ionosphere is a well developed technique for probing conditions within the waveguide. This paper seeks to review the current understanding of lightning discharge associated processes that lead to changes in the characteristics of the waveguide and thus variations in the received phase and/or amplitude of VLF transmissions. Particular emphasis is placed upon events which appear to be produced directly by lightning discharge processes. These include VLF sprites and early Trimpi for which signi1cant research e2orts have been made over the last 10 years. The properties and interpretation of these events are discussed in detail. QE-1eld produced early Trimpi are due to relatively large disturbed regions (horizontal extent ∼100 km) in the lower ionosphere. VLF sprites are produced by red sprite discharges, leading to relatively small, dense, ionospheric modi1cations made up of 1ne-scale horizontal structures (∼300 m) that stretch over wide altitude ranges (6 50–80 km). Di2erences in the ionospheric modi1cations electron temperatures and ionisation structure produces the di2ering delay times and scattering patterns in the experimentally observed VLF perturbations. Lightning-EMP produced Elves lead to large (∼500 km), relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km). Such a modi1cation should create sudden step-like changes in received VLF amplitude and phase that lack a clear relaxation signature and occur along narrow forward-scattered directions. The VLF techniques described in this article are part of the larger 1eld of research into high-altitude processes connected with thunderstorms. Lightning EMP produces a large (∼500 km), relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km). c 2003 Elsevier Science Ltd. All rights reserved.  Keywords: VLF propagation; VLF perturbation; Trimpi; Red sprite; Elve; Remote sensing; Lower ionosphere

1. Introduction In 1989 a group of researchers from the University of Minnesota testing a Low-light Level TV camera system captured the 1rst ever images (Franz et al., 1990) of the high altitude discharges now known as “red sprites”. In a scienti1c sense this paper marked the “discovery” of a new phenomenon, and sparked a great deal of interest in the regions above thunderstorms. That chance discovery of red sprites in 1989 lead to a new and vigorous area of atmospheric physics, inhabited by researchers using a wide-range of techniques. ∗

Tel.: +64-3-479-7749; fax: +64-3-479-0964. E-mail address: [email protected] (C.J. Rodger).

We now recognise that red sprites are visible to the naked eye, which has allowed a number of historical reports of short-lived high altitude phenomena to be re-evaluated (see the discussion in Rodger, 1999). However, this is not only true for optical studies. The processes which produce the luminous red sprite also lead to signatures that are detectable in other parts of the electromagnetic spectrum. As with the historic reports of optical Fashes, with hindsight we are now in a position to better understand earlier reports of atmospheric and ionospheric observations. In this paper we seek to review one such area: the use of subionospheric propagation of very low frequency (VLF) transmissions to probe the upper atmosphere, and speci1cally to investigate high-altitude modi1cation processes associated with lightning discharges.

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C.J. Rodger / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 591 – 606

The part of the electromagnetic spectrum described as VLF generally spans 3–30 kHz. Most ground-based observations in the VLF band are dominated by impulsive signals from lightning discharges. Signi1cant radiated electromagnetic power exists from a few hertz to several hundred megahertz (Magono, 1980), with the bulk of the energy radiated in the frequency bands ¡30 kHz. At VLF such pulses are termed “atmospherics”, or simply “sferics”. Except at the highest latitudes, long term noise surveys undertaken in this band contain essentially no contribution from other natural radio noise sources (Smith, 1995). In some parts of the world the VLF band is also strongly dominated by man-made radiation. Harmonics of the electrical network transmission frequencies (typically 50 or 60 Hz) can be signi1cant in much of the VLF band. From a scienti1c viewpoint such radiation is simply a source of noise, although some have suggested it may inFuence the magnetosphere (Bullough, 1995), although this has been disputed (Rodger et al., 2000). At frequencies ¿10 kHz man-made transmissions from communication and navigation transmitters can be observed in almost every part of the world. In this paper we will concentrate on observations made using such transmitters as probes of the upper atmosphere. This is not at all indicative of the total scienti1c use of observations made in the VLF band—the interested reader is directed to the recent review article of Barr et al. (2000) for a broader description of studies involving VLF (and ELF) radio waves. Few review articles are produced totally independent of earlier reviews, and this work is no exception. In this report we shall build on and update an earlier review on phenomena occurring above active thunderstorms (Rodger, 1999). Since our earlier review, it has become increasingly clear that VLF Sprites and QE 1eld produced early Trimpi have di2erent observed properties. Such distinctive di2erences are due to di2erences in the associated ionospheric changes. It has become increasingly clear that there are multiple ways to produce non-classic Trimpi perturbations. The current paper seeks to review the present understanding of lightning discharge associated processes that lead to changes in the characteristics of the waveguide and thus variations in the received phase and/or amplitude of VLF transmissions. 2. Subionospheric propagation of VLF transmissions 2.1. VLF transmitters A number of nations currently operate large VLF transmitters, primarily for communication with military submarines. While such transmitters originally served some commercial and governmental purpose, particularly for those nations involved in the building and maintenance of far-Fung colonial empires (Byron, 1996), permanent operation is now usually undertaken only for military communications. These users

continue to value the huge areas which can be covered from a single transmitter (¿10 Mm), as well as the skin depth implications o2ered by VLF frequencies (extremely important in the age of ballistic missile submarines). These bene1ts overcome the prime limitations of such transmitters: small usable bandwidths (∼20–150 Hz), and low transmitter radiation eJciencies (∼10–20%; Watt, 1967). In addition to communication stations, there have also been networks of navigation beacons operating in the VLF band. Examples are the world-wide Omega network broadcasting between 10.2 and 13:6 kHz (which ceased operation at 0300UT, 30 September 1997) and the Russian “Alpha” network from 11.9 to 14:9 kHz. 2.2. VLF transmissions as remote sensing probes Most of the energy radiated by these large VLF transmitters is trapped between the conducting ground (or sea) and the lower part of the ionosphere, which forms the Earth-ionosphere waveguide. These transmissions are said to be propagating “subionospherically”. While the creation and operation of VLF transmitters is generally undertaken by various countries’ military forces, the scienti1c use of the transmissions from these stations has been recognised for some time (see the discussion in Barr et al., 2000). Because of the frequencies at which these transmitters broadcast, their high radiated power, and their nearly continuous operation, they are extremely well suited to probing the lower ionosphere. In addition a very small percentage of the energy radiated by the transmitter can “leak” through the ionosphere into the magnetosphere and travel roughly along geomagnetic 1eld lines into the conjugate hemisphere. Such signals are also used as scienti1c probes, with modern techniques making use of cross-correlation detection to monitor VLF transmissions which have travelled in the whistler-mode through the magnetosphere (see Thomson et al., 1997, and references therein). VLF subionospheric signals reFect from the D-region of the ionosphere (the part of the ionosphere below ∼90 km). Because of its relative inaccessibility the D-region of the ionosphere is among the least studied regions of the Earth’s atmosphere. Such altitudes (∼70–90 km) are far too high for balloons and too low for most satellites, making in situ measurements extremely rare. Rocket lofted experiments have taken place in the D-region (e.g., Friedrich et al., 1990), but provide limited coverage. Radio soundings made at frequencies ¿1 MHz (e.g. ionosondes), while successful for observing higher parts of the ionosphere, generally fail in the D-region because of the low electron number densities leading to weak return signals, particularly at night. Measurements of the amplitude and/or phase of VLF transmissions have provided information on the variation of the D-region, both spatially and temporally. A schematic of subionospheric propagation is shown in Fig. 1.

C.J. Rodger / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 591 – 606

Fig. 1. Schematic of subionospheric VLF propagation. VLF transmissions propagate in the waveguide formed by the Earth and the lower edge of the ionosphere (for night time ∼85 km).

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tail below. Recently transient VLF perturbations (lasting ∼5 min) were used to study the ionospheric changes from a gamma ray Fare produced by a neutron star 23,000 light years away (Inan et al., 1999). Maximum changes of ∼24 dB in amplitude and ∼65◦ in phase were observed on the transmitter-receiver path from NPM (Hawaii) to Palmer (Antarctica). This amplitude change is of the same order as the di2erence between the daytime and nighttime ionosphere, when observed using subionospheric VLF propagation. At the far extreme, a 1.4-megaton thermonuclear detonation (Star1sh test) at 400 km altitude has also been observed to produce VLF perturbations (maximum change of ∼175◦ in phase) by altering the nighttime ionosphere (Zmuda et al., 1963). 2.5. Theory of VLF wave propagation

2.3. Day-time variations in subionospheric propagation Variations in the ionospheric D-region lead to changes in the propagation conditions for VLF waves inside the waveguide, and hence changes in the observed amplitude and phase of VLF transmissions. Variations in time and space in the lower part of the waveguide can also lead to changes in VLF propagation conditions (e.g. reFections from mountain ranges (Barr and Armstrong, 1996)). However, for a 1xed transmitter–receiver combination variations over time in the “earth plate” of the waveguide have not (thus far) been shown to produce detectable changes in VLF propagation. For daytime propagation conditions the D-region is particularly stable, with reFection heights occurring at about 70 –75 km, the variation being strongly dominated by the change in Lyman- Fux with solar zenith angle (McRae and Thomson, 2000). Additional modi1cations are driven by solar Fares (Thomson and Clilverd, 2001) and total solar eclipses (Clilverd et al., 2001). Longer-term changes in the lower ionosphere also alter VLF propagation parameters, and include modi1cations driven by solar cycle changes (Thomson and Clilverd, 2000). 2.4. Night-time variations in subionospheric propagation Night-time propagation at VLF frequencies is less stable and predictable than for day-time paths, although suJcient for communications purposes. The di2erence in stability reFects short-term variation in the night-time D-region and the lack of a dominant energy source (c.f. the Sun in daytime). ReFection heights occur at about 80 –90 km altitude. Much scienti1c attention has focused on short time-scale (∼100 s) modi1cations of the D-region leading to VLF phase and amplitude perturbations, particularly those produced by red sprites, electromagnetic pulses (EMP) from lightning (Rodger, 1999, and references therein), changes in quasi-electrostatic (QE) thunderstorm 1elds (Inan et al., 1996c), and whistler-induced electron precipitation (Helliwell et al., 1973). These are discussed in more de-

In order to understand the received subionospheric transmissions signi1cant theoretical e2ort was required to describe the propagation of VLF radio waves inside the Earth ionosphere waveguide. The earliest e2orts in this area were focused on undisturbed ionospheric conditions. For short ranges (¡1000 km) a ray theory approach is appropriate, including the “multiple-hops” of rays which reFect from the ionosphere as they propagate to the receiver. For longer ranges a waveguide mode formulation is favoured, treating the Earth and ionosphere as plates of a parallel plate waveguide. In this formulation the 1eld at any point in the guide is then derived in terms of waveguide modes. The interested reader is directed to the classic texts of Budden (1961) and Wait (1996) (the later a reissue of a 1962 text). A detailed description of the historic development of such approaches has been presented (Barr et al., 2000). This work eventually leads to sophisticated computer programs to solve the modal equation for a waveguide comprised of an inhomogeneous, anisotropic ionosphere and a ground of 1nite conductivity which then provides modal summations along a path over which the ionosphere and ground parameters changed with propagation distance. Developed by the United States Naval Oceans Systems Center (NOSC), the most developed of these numerical models is the Long Wave Propagation Capability (LWPC) program (Ferguson and Snyder, 1987). Building on NOSC programs, numerical models have been developed to model the e2ects of localised disturbances in the earth-ionosphere waveguide, particularly focused on the Trimpi phenomena discussed below (e.g., Poulsen et al., 1993; Nunn, 1997). While computationally eJcient, these codes have some limitations. For example, these programs have the disadvantage of being inaccurate at very high altitudes ¿ 90 km, requiring either the inclusion of an additional compensation factor (Clilverd et al., 1999), or making use of a full wave VLF analysis (e.g., Yagitani et al., 1994). The later is an example of a mathematical technique for modelling subionospheric VLF propagation which does not reply upon the wave-mode (or ray) formulation. Other examples are the Finite Element Method (Baba and

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Fig. 2. Phasor diagram for a phase perturbation of  and amplitude perturbation of QA relative to the unperturbed wave (signal immediately before the perturbation). From these the amplitude (|M |) and phase () of the scattered wave is obtained from phasor subtraction.

Hayakawa, 1996), and the Finite-Di2erence Time-Domain method (Poussard and Corcu2, 2000). To the best of the author’s knowledge, studies to date of red sprite characteristics using subionospheric VLF propagation have made use of only wave-mode and ray theory. 3. Perturbations on VLF transmissions It has become standard practice to describe short time-scale perturbations (∼100 s) on sub-ionospheric transmissions in terms of a phasor diagram (Dowden and Adams, 1988), as shown in Fig. 2. Phasor subtraction of the unperturbed signal (immediately before onset), from the perturbed signal enables calculation of the phase and amplitude of the scattered signal relative to the phase and amplitude of the direct unperturbed VLF transmission. Prior to the introduction of the phasor diagram method it was common to discuss the changes observed in the perturbed signal itself (and often in amplitude only), which are rarely more than 1dB (Inan et al., 1993). The large changes mentioned above (from the e2ects of a gamma-ray Fare, thermonuclear explosion, and the diurnal variation) are the directly observed changes, rather than the “scatter phase” and “scatter amplitude” found through the phasor approach (also known as the “echo phase” and “echo amplitude”). 4. Classic Trimpi VLF perturbations Short time-scale (∼100 s) perturbations in the phase and/or amplitude of subionospheric VLF transmissions are known as “Trimpi” perturbations. Trimpi were 1rst recognized by M.L. Trimpi during his time as a 1eld scientist in Antarctica. The perturbation began with a relatively fast (∼1 s) change in the received amplitude and/or phase, followed by a slower relaxation (¡100 s) back to the unperturbed signal level. This 1rst type of Trimpi perturbation recognised is now termed a “classic Trimpi”. Classic Trimpi perturbations are caused by whistler-induced electron precipitation (WEP) from the Van Allen radiation belts increasing the electron concentration in the

night-time D-region and hence altering the properties of the Earth-ionosphere waveguide (Helliwell et al., 1973; Rycroft, 1973; Strangeways, 1996). The energetic electron precipitation arises from lightning produced whistlers (Storey, 1953) interacting with cyclotron resonant radiation belt electrons in the equatorial zone (e.g., Tsurutani and Lakhina, 1997). A schematic diagram of this situation is shown in Fig. 3. The beginning of the VLF perturbation occurs ∼0:6 s after the associated sferic (Armstrong, 1983), depending on the geomagnetic latitude; this delay is consistent with the time required for a whistler to propagate through the ionosphere and magnetosphere to the geomagnetic equator, interact with energetic electrons in the radiation belts, and for these electrons to arrive at the D-region as WEP. Due to their production by WEP, classic Trimpi are also sometimes referred to as “WEP Trimpi”. An example of a classic Trimpi perturbation is shown in Fig. 4. Both communications transmitters, and later navigation beacons (e.g., Inan et al., 1985), have been used to observe Trimpi. As classic Trimpi are observed only during night-time ionospheric conditions (Leyser et al., 1984), it is believed that the WEP is not energetic enough to make signi1cant changes to the day-time ionosphere (the electron density at 80 km is roughly one thousand times larger by day than by night). Classic Trimpi permit observers to study energetic electron precipitation from the radiation belts, and the chemistry of the lower ionosphere, from locations remote from the actual precipitation region. Recent calculations suggest that WEP is an important process in determining the life-times of radiation belt electrons (Abel and Thorne, 1998). Until recently there has been signi1cant uncertainty as to the typical size of the D-region patch altered by WEP, the typical levels in ionisation enhancement present in the patch, the dimensions of plasmaspheric ducts in which whistlers are guided, and the electron precipitation Fux. There is now growing evidence that the spatial extent of the WEP modi1ed D-region, which is generally referred to as a “patch”, is large (hundreds of kilometres or greater). Most classic Trimpi events observed display phase increases (phase advance) and amplitude decreases (Smith and Cotton, 1990; Clilverd et al., 1999), the so-called (+; −) perturbations (Dowden and Adams, 1988), as seen in Fig. 4. Such perturbations are best explained in terms of a large ionospheric disturbance straddling the great circle path (GCP) between the transmitter and receiver. Under certain conditions, modal interference or di2raction can change the sign of these perturbations (Lohrey and Kaiser, 1979; Dowden and Adams, 1989), although this should be less signi1cant for the long paths from northern hemisphere transmitters to Antarctica where many observations have been made. Small patches of ionisation distributed over a range of distances from the GCP would be expected to produce Trimpi events with a variety of phase and amplitude relationships (i.e. (+; +), (−; −), etc.) (Dowden and Adams, 1989). A single large ionospheric disturbance

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Fig. 3. Schematic of VLF perturbation produced by whistler induced electron precipitation (WEP). Trimpi perturbations in VLF transmissions are caused by WEP altering the properties of the Earth-ionosphere waveguide.

Fig. 4. Example of a classic Trimpi perturbation observed on transmissions from the US Navy transmitter NPM (Hawaii; 23:4 kHz) at Faraday (Antarctic Peninsula) on 23 April 1994. In this case the amplitude (sold line) decreases, while the phase (dotted line) increases (reproduced by permission of American Geophysical Union ? 2001; Dowden et al., 2001a, b).

located o2 the GCP will also lead to a variety of phase and amplitude relationships, as shown from work on ionospheric depressions caused by nuclear explosions (Crombie, 1964) and con1rmed by wave-mode formulations. In order to estimate the size of WEP modi1ed patch in the D-region the conclusions above need to be combined with observations from networks of receivers observing multiple transmitter paths (e.g., Inan et al., 1991; Dowden and Adams, 1993; Chen et al., 1996). Most recently, classic Trimpi signatures have been examined from several locations on the Antarctic Peninsula using multiple northern hemisphere VLF transmitters. Ionospheric patch sizes caused by WEP were found to be large, i.e., at least 600 km × 1500 km, with the longest axis orientated east-west (Clilverd et al., 2002). There has been considerable controversy in the analysis of VLF perturbations and the properties of the WEP

modi1ed D-region inferred from Trimpi observations and modelling. For example, some reports suggested that observations fell into two types. Firstly, events consistent with a smooth lateral variation in patch properties and with large spatial dimensions, as outlined above. Secondly, patches thought to be located well o2 the transmitter-receiver GCP, with small spatial dimensions (¡100 km), and 1ne structure with dimensions of 10 km or less (Dowden and Adams, 1988, 1993). The interpretation of these early studies are made more complex by the realisation that non-WEP produced Trimpi (at least some of which are associated with red sprites) were “contaminating” these observations, likely leading to the second set of patch properties outlined above. The vertical extent of WEP patches has not received the same intensity of examination as the spatial extent. Comparing the maximum perturbation levels observed in two classic Trimpi events with the perturbation calculated using various WEP-modi1ed electron density pro1les, Lev-Tov et al. (1995) identi1ed pro1les which were most consistent with the data (although not the same pro1le for the two cases). A di2erent approach has also been presented, using the experimentally observed decay timescale of Trimpi perturbations to estimate the vertical dimensions of WEP modi1ed regions (Dowden et al., 2001a, b). WEP patches were found to be thin (signi1cant ionisation distributed over ∼20 km vertically), which is not dissimilar to the range identi1ed by Lev-Tov et al. (1998). We have used signi1cant space describing the observations of classic Trimpi, and the experimental parameters which have lead to the conclusion that they are caused by spatially large regions with relatively small increases in D-region ionisation, over a fairly limited vertical extent. This has been undertaken to shed light on the di2erences with other VLF perturbations discussed below.

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5. Early Trimpi VLF perturbations A decade after the 1rst report of the classic Trimpi, a new type of VLF perturbation was presented which had very small time delay between the lightning discharge and the onset of the perturbation (Armstrong, 1983). The original Trimpi studies made use of data routinely analysed to 2 s resolution, whereas by this time 100 ms resolution data was available, showing that the new type of Trimpi occurred ¡100 ms after the associated lightning discharge. This class of Trimpi has become known as “early” Trimpi (Inan et al., 1988) as they occur before one would expect the onset of a classic Trimpi. An example of an early and classic Trimpi is shown in Fig. 5. The majority of early Trimpi have the same visual decay form as a classic Trimpi, the signi1cant di2erence between the di2erence in onset times, relative to the associated lightning discharge. The original observations reported by Armstrong were made in the Antarctic where lightning activity is extremely uncommon (Ingmann et al., 1985). It is assumed that the change in the waveguide that produced these early Trimpi events observed occurred far from the receiver (located at Siple station, Antarctica), possibly near the transmitter in the northern hemisphere. Antarctic studies of classic Trimpi exploit this idea to discriminate between Trimpi types when complementary sferic data are unavailable. By selecting only Trimpi events that are simultaneously observed on multiple transmitter-receiver GCPs, events that are local to the Antarctic receiver can be identi1ed, and hence the Trimpi limited to those caused by WEP (e.g., Dowden et al., 2001b; Clilverd et al., 2002). Non-classic Trimpi appear to be produced through several processes, all associated with lightning discharges, and leading to VLF perturbation events with somewhat di2erent properties. In the following section we will

summarise the analysis around early Trimpi produced by changing quasi-electrostatic (QE) thundercloud 1elds, by VLF Sprites, and by VLF Elves. Prior to the identi1cation of these mechanisms, other attempts were put forward to explain the observations of VLF perturbations immediately associated (in time) with lightning discharges. These include: vertical transport of charge upward into the ionosphere above the thundercloud (Armstrong, 1983), second order cyclotron resonance e2ects (between electrons and lightning induced whistler-mode waves) causing pitch angle scattering of energetic electrons into the loss cone (Neubert et al., 1987), and lightning produced electric 1elds lowering the mirror point height of 50 keV (magnetospheric) electrons from about 150 km to D-region altitudes (Inan et al., 1988; Burke, 1992). 6. Thunderstorm QE-'eld produced early Trimpi Strictly speaking, this is the only class of non-WEP associated VLF perturbations which should be referred to as “early Trimpi” events. These events have been de1ned as VLF perturbations that exhibit a rapid onset (¡20 ms between causative sferic and the beginning of the perturbation), are associated with amplitude changes of magnitude ¿ 0:2 dB, and lasting for longer than 10 s (e.g., Inan et al., 1996a). A signi1cant amount of research has been undertaken into early Trimpi identi1ed using the de1nition given above (or slight variants upon this de1nition), primarily by Inan and co-workers at Stanford University. The detection threshold (∼0:2 dB) is determined by the background VLF radio atmospheric noise levels (Inan et al., 1996a) while the onset time parameter (¡20 ms) is likely due to a combination of equipment time resolution and the intrusion of sferic energy

Fig. 5. An example of a WEP produced classic Trimpi (left), and an early Trimpi (right) amplitude perturbation (shown in arbitrary linear units). The associated sferic amplitude data is shown in the lower panel, indicating the lack of a clear time delay between the sferic and onset of the early Trimpi perturbation (reproduced by permission of American Geophysical Union ? 2000; Sampath et al., 2000).

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into the band around the frequency of the VLF transmitter (e.g., 50 Hz sampling rate for VLF amplitude and 10 Hz for phase (Sampath et al., 2000)), leading to “ringing”. Early Trimpi are, by de1nition, associated with sferics, and therefore, lightning discharges. Using data from the United States National Lightning Detection Network (NLDN) (Wacker and Orville, 1999), which provides spatial and temporal information on CG lightning discharges in the continental USA, it was found that only 20% of early Trimpi observed were correlated with Cloud to Ground lightning discharges (Inan et al., 1993). At the time of the study the NLDN detection rate was estimated to be about 70 –80%, more than suJcient to detect the majority of CG discharges. Early Trimpi which were CG correlated were regularly associated with negative CGs, and many of these discharges had low peak return currents, in some cases as small as 20 kA. Cloud to ground discharges with peak currents ¿20 kA are very common (¿50% of all CG discharges (Ogawa, 1995)). Up to this point attempts to explain early Trimpi events relied upon large electric 1eld amplitudes produced by high currents in the lightning discharge (e.g., heating of ionospheric electrons by the VLF EMP from lightning (Inan et al., 1991)), and could not explain such low thresholds. It appears that the “missing” early Trimpi associated discharges which created the observable sferics were not CG lightning but rather cloud-to-cloud or intra-cloud (within the thunderstorm) lightning discharges (Johnson and Inan, 2000). However, it appears that another factor (additional to lightning activity) is required to explain why only some low current lightning events produce early Trimpi. There is a large body of evidence indicating that the ionospheric disturbances which produce early Trimpi must lie within 50 km of the transmitter–receiver great circle path, with the associated lightning discharge located under the disturbance (Inan et al., 1993, 1995b, 1996a–c; Johnson et al., 1999). Making use of a nine receiver array of closely spaced (∼65 km) stations, early Trimpi were found to have forward scattering patterns exhibiting 15 dB beamwidths of less than 30◦ (i.e., scattering strongly along the transmitter– receiver GCP only), consistent with a disturbed region in the lower ionosphere with an horizontal extent of 90 ± 30 km (Johnson et al., 1999). No early Trimpi event observed by this array has been identi1ed having signi1cant wide-angle scattering, as would be expected from a small, dense, ionospheric modi1cation. As such, this array does not appear to have detected the VLF perturbations associated with red sprites (see Section 7.0). The recovery signatures of early Trimpi and classic (WEP) Trimpi have recently been contrasted. A comparison of 15 events of each type indicates that the majority of early Trimpi recover more rapidly towards pre-event levels during the 1rst 20s of the perturbation than do WEP Trimpi (Sampath et al., 2000). While based on a rather limited number of events, it also appears that early Trimpi events have a wider range of recovery times (the time for the

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Fig. 6. Schematic of a proposed mechanism leading to early Trimpi perturbations. Changes in the quasi-electrostatic 1elds produced by the charging and discharging of thunderstorms lead to heating and conductivity changes in the upper atmosphere above the storm (reproduced by permission of American Geophysical Union ? 2000; Sampath et al., 2000).

perturbation to decay back to pre-event levels) than classic Trimpi (∼60–240 s compared to ∼120–180 s). There appears to be a clear di2erence between the recovery patterns of the two perturbations, suggesting that the physical parameters of the ionospheric disturbance are di2erent. On the basis of the experimental observations outlined above, it has been suggested that these early Trimpi may not necessarily involve ionisation changes and that the recoveries represent the decay or build up of thunderstorm charges after a lightning discharge (Inan et al., 1996c). A schematic of the proposed mechanism leading to early Trimpi perturbations is shown in Fig. 6. Immediately after a lightning discharge transient quasi-electrostatic (QE) 1elds with high intensities exist at high altitudes above thunderstorms, leading to conductivity changes over large areas through the heating of ionospheric electrons. For particularly intense discharges the QE 1elds appear to be intense enough to cause break-down and produce the highly structured ionisation observed in red sprites (Pasko et al., 2000). After these transients, however, relatively stable thunderstorm charge distributions are suggested to produce ionospheric QE 1elds that maintain the ionospheric electrons at a persistently heated level well above their ambient thermal energy. Changes in the thundercloud charge (those involved with lightning discharges) lead to heating or cooling above or below this level, and would be registered as sudden subionospheric VLF signal changes, occurring simultaneously with lightning discharges, and thus producing the early Trimpi perturbation. The recharging of the thunderstorm returns the ionospheric electrons to their previous levels and thus produces the decay of the observed perturbation. As the mechanism relies upon on relative changes in the QE 1elds, discharges of either polarity or direction (CG or cloud-to-cloud) might produce a change in the

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lower ionospheric boundary suJcient to generate an observable VLF perturbation. In addition, the creation of a VLF perturbation also depends on the background level, thereby explaining how small lightning discharges can produce an early Trimpi in some, but not most, situations (Fig. 6). 7. VLF perturbations associated with red sprites The upper atmospheric processes associated with luminous red sprites have also been shown to produce nonWEP Trimpi perturbations on VLF transmissions. Due to the nature of this special issue, we will discuss the VLF perturbations associated with sprites in detail. However, we will not present the properties of red sprites (and related phenomena). The interested reader is directed towards the other papers in this special issue, and, for historic overviews, the review by Rodger (1999) and an earlier special issue (“E2ects of Thunderstorm Activity on the Upper Atmosphere and Ionosphere”, Journal of Atmospheric and Solar– Terrestrial Physics vol. 60, numbers 7–9, 1998). The VLF perturbations associated with red sprites have been termed “VLF sprites” (Inan et al., 1995a; Dowden et al., 1996a–c), or “CID Trimpi” (Dowden et al., 2001b), where CID refers to “Cloud-Ionosphere Discharges”, one of the terms used to describe red sprites (e.g., Winckler, 1995). Although Inan et al. (1995a,b) reported that only a small number of very large and intense red sprite events were associated with VLF perturbations, there is now strong evidence that they are well correlated phenomena (Dowden et al., 1996a–c; Dowden et al., 1998; Hardman et al., 1998). A signi1cant amount of research has been undertaken into VLF sprites, primarily by Dowden and co-workers from New Zealand. These authors have made subionospheric observations using the OmniPAL (and upgraded “AbsPAL”) instruments which reject sferic-noise by tracking the amplitude and phase of the VLF transmissions (Dowden et al., 1998). As shown by Dowden et al. (2001b), these receivers are largely immune from sferic-noise, thus with inherent limitations to the speed at which the receiver can react to changing transmissions. VLF sprites share some similarities with early Trimpi (described above), in that they are transient VLF perturbations lasting of order minutes, with extremely small delays between the lightning discharge and the beginning of the VLF perturbations (a very small “onset”). However, VLF sprites do not share many important properties with early Trimpi, and in particular often do not meet the de1nition of an early Trimpi described in Section 6. The time delay between the lightning discharge and the beginning of the VLF perturbation is generally greater than 20 ms (Dowden et al., 2001a,b). This is to be expected, as the cooling time for the hot electrons in red sprites (Green et al., 1996) is of this order (or more) for high altitudes, making the sprite plasma transparent to VLF until signi1cant cooling has taken place.

In addition, some red sprites appear to occur with signi1cant delays from the associated CG lightning. The scattering patterns of the ionospheric disturbances leading to VLF sprites are also clearly di2erent from those leading to early Trimpi. While the largest perturbations are observed for red sprites located on the transmitter-receiver great circle path (Hardman et al., 1998), detectable perturbations are also observed for sprite-events located well o2 the GCP, as long as the sprite lies within ∼500–1000 km of the receiver (Rodger et al., 1999). The experimental evidence suggests that essentially all nearby red sprites will produce VLF sprites irrespective of their displacement from the Great Circle Path (GCP) (Dowden et al., 1996a,b,c; Dowden et al., 1997; Hardman et al., 1998), and that red sprites located near the GCP can be at considerable distances from the receiver. A schematic of scattering of VLF by red sprites is shown in Fig. 7. This 1nding implies ionospheric conductivity changes that are relatively small and “dense”. Such large-angle scattering has never been observed for early/fast events (Johnson et al., 1999). These properties suggest little overlap between early events and VLF Sprites. Having identi1ed the wide scattering pattern of VLF transmissions by red sprite associated ionospheric changes, this was used to show that VLF sprites are strongly correlated to red sprites in a “blind test”. After identifying 24 VLF sprites, the timing of the events was compared with red sprites observed over that period. All of the perturbations corresponded to optical red sprites, and only 1 (very faint) sprite event was noted which was not associated with a VLF perturbation showing wide scattering patterns (Dowden et al., 1996b). 7.1. Structure in red sprite ionisation In a series of experiments in the United States, and later Australia, it has been shown that the ionospheric modi1cation associated with red sprites leads to scattering of VLF over a wide range of angles. In the extreme case red sprites can cause strong wide-angle scattering to angles as high as 180◦ (i.e., back towards the VLF transmitter) (Dowden et al., 1996a). An example of this is shown in Fig. 8, where a sprite located south of the Yucca Ridge receiver caused an ionospheric modi1cation that backscatterd VLF transmissions from NLK (Seattle), causing a VLF perturbation observed at Yucca Ridge(in this case perturbations were also seen on NAA and NSS). These observations imply that the scattering source is structured enough (on ∼1 km scales) to cause signi1cant backscatter, suggesting that the luminous structure seen in red sprites (in some cases, quasi-vertical columns) are present in the electrical structure, and that red sprites produce electrical conductivities signi1cantly di2erent from their surroundings. Using multi-receiver arrays, it has been shown that the scattering pattern from red sprites has a strong frontal lobe, in addition to signi1cant large-angle scattering (Hardman et al., 1998). This is consistent with the scattering pattern produced by a theoretical model of the sprite as an array of interacting

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Fig. 7. Schematic of VLF scattering from a cluster of vertical plasma columns to produce a VLF perturbation at the receiver. As the sprites have a complex electrical structure, there can be signi1cant scattering o2 the great circle path (GCP), right up to 180◦ (Reproduced with c 1999, Rodger, 1999). permission of American Geophysical Union


Fig. 8. Example of wide angle VLF scattering from a Sprite event, observed during Sprites ’95. The paths from the transmitters are shown as dashed lines, the scattered signals to the receivers as full lines (Modi1ed with permission of American Geophysical Union ? 1996; Dowden et al., 1996a).

vertical plasma columns (e.g., Rodger et al., 1998b; Rodger and Nunn, 1999), indicating that some part of the sprite is “hard” and contains complex structure. An example of these calculations is shown in Fig. 9, where the VLF scattering pattern for NLK has been calculated using the experimentally determined structure of a columniform-sprite (Wescott et al., 1998). Unfortunately, one cannot use the VLF scattering pattern to determine the speci1c distribution of 1ne structure contained in the sprite (Rodger et al., 1998a). The scattering interaction between di2erent parts of the structure means that some parts can be e2ective “shielded out”, and do not play a part in the scattering process. Signi1cant 1ne structure down to ∼30 m has been observed in red sprites (Gerken et al., 2000), although the dense 1ne structure seen

in some red sprites is unlikely to be resolved using VLF wavelengths (∼10 km). Although not clearly identi1ed as such, at least some VLF perturbations observed in Europe are likely to have been caused by red sprites. Corcu2 (1998) monitored the British transmitter GBR from a receiving point in France, and compared observed perturbations with CG discharges collected by a lightning detection and location system. The experimental setup is shown in Fig. 10. A total of 6 perturbations observed on GBR were associated with simultaneous VLF sferics, 5 of which were positive CG lightning (the sixth was not associated with any CG). However, 3 of these 5 early Trimpi on GBR occurred simultaneously with VLF perturbations on the French transmitter HWU, located at right-angles to the GBR-receiver GCP. This is highly suggestive of the wide-angle scattering identi1ed as a characteristic of VLF sprite perturbations. Note that while the modelled scattering pattern indicates signi1cant wide-angle scattering, it is not an isotropic scattering pattern. One would expect an isotropic pattern from a single conducting column (or stalactite) in the Earth-ionosphere waveguide (Wait, 1991). A structured scattering body will scatter more strongly in the forward direction, but with signi1cant side-lobes (e.g., Rodger and Nunn, 1999). 7.2. Red sprite ionisation levels As discussed above, the VLF scattering properties associated with red sprite events suggest that sprites lead to changes in the upper atmosphere that are relatively small (∼ hundreds of meters or less), clustered, and “dense” in comparison with their surroundings. It has been argued that red sprites are associated with structured plasma

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Fig. 9. The VLF scattering pattern for an experimental determined sprite-column distribution, scattering transmissions from NLK (near Seattle, 24:8 kHz). The left panel shows the distribution determined by Wescott et al. (1997) for an event which occurred on 19 June, 1995. The propagation direction for NLK is shown, as is the positive CG position (marked by a star). The right panel shows the magnitude of the scattered 1eld in dB, relative to the incident 1eld amplitude. The scattering angle of zero corresponds to forward scattering (along the GCP). Adapted from Rodger et al. (1998b).

Fig. 10. Map of the experimental setup used by Corcu2 (1998). The shaded region shows the location of the positive CG discharges which caused simultaneous perturbations on the VLF transmitters GBR and HWU.

(increases in the ionisation density) in the upper atmosphere. While conductivity changes can also be caused by electron temperature changes inFuencing the collision frequency, it has been shown that ionospheric electrons will cool within ∼100 ms, even at the highest altitudes (Rodger et al., 1998c), and so are unlikely to be signi1cant for perturbations lasting ∼100 s. The 1ndings from subionospheric VLF perturbations were amongst the earliest evidence that red sprites are highly ionised, for which the experimental evidence is now over-whelming. A number of theoretical studies have attempted to use the observed properties of VLF sprites to infer information about the (electrical) nature of the red sprites, using increasingly more complicated models. The fundamental conclusions of these studies have not signi1cantly changed with the addition of greater complexity. The observed VLF perturbations are best explained by red sprite plasma that is highly ionised (4 – 6 orders of magnitude at some heights) in comparison with the ambient night-time ionosphere (e.g., Rodger and Nunn, 1999; Nunn and Rodger, 1999). Most recently it has reported that a uniform conductivity of at least 30 S=m is required to produce the observed strength of scattered signals. This corresponds to an electron density at 70 km altitude of ∼104 electrons per cc (∼1010 m−3 ) and so about 105 =cc at 55 km, over 6 orders of magnitude above the ambient density at 55 km (Dowden et al., 2001a). While these ionisation levels are clearly very large, photometric data now con1rms signi1cant ionisation in at least some

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Fig. 11. Superposition of 12 VLF sprites, showing the clear logarithmic decay over a large time and phasor magnitude range (reproduced with permission of American Geophysical Union ? 2001; Dowden et al., 2001b).

red sprite events (Armstrong et al., 2000), with energetic “carrot” sprites appearing to exhibit ionisations between 104 and 105 electrons per cc (1010 and 1011 m−3 ). The streamer mechanism which appears to lead to the 1ne-scale structure in optical red sprites (Pasko et al., 1998) should lead to even higher ionisation levels (V. Pasko, personal communication, 1999), albeit on ∼10 m scales. 7.3. Vertical structure of red sprite ionisation It has been shown that the magnitude of a VLF sprite perturbation (the magnitude of the phasor of the wave scattered o2 the sprite plasma, which, when added to the unperturbed phasor of the VLF transmission, makes the phasor of the perturbed wave, as shown in Fig. 2) decays logarithmically with time (Dowden et al., 1997, 1998) as the perturbation recovers to pre-event levels (typically 30 –100 s). The consistency of this decay pattern is shown in Fig. 11, where the relaxation of 12 VLF Sprites are shown, superimposed after normalisation to a common scatter-magnitude and decay-time. This logarithmic decay-signature is expected for ionisation spread over a wide range of altitudes; for example, by an ionised column in the Earth-ionosphere waveguide that becomes progressively shorter as the ionisation relaxes back to ambient conditions (Dowden and Rodger, 1997). The time dependence of VLF sprites has been used to determine the vertical range of red sprite ionisation. From simple models of the relaxation of the red sprite plasma and the scattering of VLF transmissions (Dowden and Rodger, 1997), the vertical length of the sprite plasma column decreases from the bottom up as the logarithm of time (and therefore, so does the scatter magnitude). These studies have shown that there is signi1cant red sprite ionisation at altitudes extending from 6 50 to ∼80 km altitude. However, great care needs to be taken in using VLF techniques to establish the lower bound of red sprite ionisation. As has

Fig. 12. Relaxation with time of a suggested red sprite electron density pro1le (reproduced with permission of American Geophysical Union ? 1999; Nunn and Rodger, 1999).

been previously noted, the minimum time resolution for VLF perturbation studies is ∼100 ms, where as optical evidence indicates the red sprite ionisation impulse lasts for only ∼150 s (Armstrong et al., 2000). In the 1rst ∼100 ms of a red sprite event, almost all ionisation located below ∼50 km altitude will disappear through relaxation processes (Nunn and Rodger, 1999), as shown in Fig. 12. This modelling also indicates that it may be diJcult to detect the long-lived (¿ 60 s) upper-most portions of the red sprite plasma (¿ 80 km), as the scattered VLF will be small in comparison with the typical noise levels. 8. VLF perturbations associated with Elves As noted above, the lightning electromagnetic pulse was at one stage a candidate mechanism to explain early Trimpi events through heating of ionospheric electrons (Inan et al., 1991, 1993). This was later discarded due to the strength of the discharges associated with these events. More recently, the electromagnetic pulses (EMP) from intense lightning discharges have been found to produce short-lived luminous events in the lower ionosphere, termed Emissions of Light and VLF perturbations due to Electromagnetic Pulse Sources (Elves) (Fukunishi et al., 1996). Elves last less than 1 ms, occur at 75 –105 km altitude, and have a horizontal width of up to 700 km (Barrington-Leigh and Inan, 1999). While it was originally reported that elves are always accompanied by “large amplitude VLF perturbations” (Fukunishi et al., 1996), initial modelling has cast some doubt as to whether elve-associated alterations of the lower-ionosphere could lead to such perturbations (Lev-Tov et al., 1998).

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Examples of these VLF perturbations have not been presented in the literature. The strong EMP which creates elves clearly produces light through heating of high-altitude electrons. The consequential modi1cation in the conductivity at the base of the ionosphere should signi1cantly perturb subionospheric VLF propagating under it (Dowden et al., 1991). However, such a heating e2ect would last only a few ms (Dowden et al., 1991). Theoretical modelling indicates that suJciently strong lightning EMP also leads to changes in ionisation (Cho and Rycroft, 1998), which at elve-altitudes will be relatively long lasting (Rodger et al., 2001). Lightning EMP produces a large (∼500 km), relatively smoothly varying ionospheric disturbance at high altitudes (∼85 km), near the nighttime VLF reFection height. Subionospheric VLF perturbations associated with these disturbances would display narrow forward scattering along the GCP (Wait, 1964). The relaxation of such perturbations to pre-event levels would be expected to be extremely slow due to the long lifetimes of electrons at elve-altitudes. Such perturbations would likely appear as sudden step-like changes in received amplitude and phase without a clear relaxation signature. The decay of these perturbations are likely to be masked by other variations in the subionospheric signal occurring over hundreds of seconds. Perturbations of this type have been occasionally reported (e.g. Inan et al., 1988), although they have not been linked to elves. At this stage it has not been conclusively shown that lightning EMP produced elves that are associated with VLF perturbations, although the term “VLF elves” has been put forward to describe such events (Dowden, 1996). Although lightning-EMP e2ects appear to be fairly well understood, the secondary issue of e2ects on long-wave propagation inside the Earth-ionosphere waveguide has received little attention. Modelling using full wave or 1nite element method propagation descriptions (Section 2.5) should be able to clarify this issue with relative ease, even if the modelling was limited to two dimensions. Given that expected step-function characteristic of VLF elves, they should be easy to classify in subionospheric data, and hence allow long-distance sensing of elve activity. 9. RORDS In the text above we have discussed in detail observations and analysis of transient perturbations on subionospheric VLF transmissions which have time-scales of ∼100 s. A shorter lived perturbation, associated with lightning, has also been observed (Dowden et al., 1994; Inan et al., 1996a). It is characterized by a very small delay (¡20 ms (Inan et al., 1996a)) between lightning and the beginning of the perturbation (i.e. “early”), but also has a rapid decay (∼1 s) compared with the various types of Trimpi perturbation (e.g., classic, CID, early, etc). These characteristics led to the acronym rapid onset, rapid decay (RORD) to describe these

perturbations (Dowden et al., 1994). An early explanation for VLF sprites required a superimposed RORD (presumably sprite-produced) upon a classic Trimpi (Dowden et al., 1994). These authors have withdrawn this suggestion on the basis of observations undertaken at very low magnetic latitude where there is no possibility of WEP producing classic Trimpi (Dowden et al., 1997). Calculations have shown that the “short-duration events” of Inan et al. (1996a) (essentially identical to RORDs) are consistent with the VLF response to electron temperature changes (without signi1cant ionisation changes) due to heating by QE 1elds (Pasko et al., 1995; Inan et al., 1996a). It has been speculated that the RORD perturbations reported by Dowden et al. (1997), which tended to occur near local noon, might perhaps be caused by red sprites occurring during the day, when only the lowest, shortest-lived parts of the sprite plasma would exist below the day-time VLF reFection height (∼75 km) (Rodger, 1999). It seems most likely that RORDs are due to non-ionising QE 1elds or perhaps to sprite ionisation located only at lower altitudes. 10. Discussion The VLF observations described above have lead to signi1cant advances in our understanding of high altitude phenomena associated with lightning discharges, and in particular high altitude optical phenomena (red sprites, etc). The techniques lend themselves to the investigation of the electrical properties of phenomena occurring at ∼60–90 km altitude, and have several positive features: the cost of the powerful VLF transmitters is wholly met by governments for non-scienti1c reasons, whereas the receiving equipment is comparatively inexpensive; a large amount of work has been undertaken to describe VLF propagation to support the transmitters’ communications functions, providing a 1rm “theoretical underpinning” for research into transient perturbations; detection of events is not limited to optically -clear viewing times; very long range detection of events is possible (and with the growing number and size of lightning detection networks in the world, these data will become more and more useful). However, VLF techniques also have some disadvantages: in some situations scattering is only signi1cant along the Great Circle Path between the transmitter and receiver, which is highly dependent upon the locations of the limited number of VLF transmitters in operation and largely beyond the experimenters control; as the transmitters are not operated for scienti1c reasons they are (largely) beyond the inFuence of researchers, and can be placed in experimental transmission modes (or even switched o2 for maintenance) at the moment a research campaign is expected to start (in some cases basic information about these transmitters has been regarded as state secrets); the long wavelengths broadcast, which allow observation at D-region altitudes, provide limited resolution to activity in some scales; because of the

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long-detection ranges possible, the location of events along the GCP can be uncertain in some situations. In addition, it is clear from the descriptions of the various “Trimpi”-like VLF perturbations that there could be considerable diJculty in correctly classifying a given perturbation. There is signi1cant disagreement between the researchers involved in this 1eld. In this paper we have attempted to describe the characteristics and interpretations of events as reported by the research groups involved, without signi1cantly favouring one over the other. It should be noted, however, that some workers report that they have not observed the wide-angle scattering which is one of the reported characteristics of VLF sprites, while other workers report perturbation characteristics which do not appear to be consistent with the QE heating mechanism for early Trimpi. Despite large numbers of detailed studies and exchanges (e.g., Dowden, 1996; Inan et al., 1996b) these are still areas of active debate. In this review we focussed strongly on studies considering variations in the received phase and/or amplitude of man-made VLF transmissions. As such we have taken the generally accepted de1nition of the VLF range. It should be noted that the manner in which VLF wave propagation can be described also applies to natural and man-made sources over a wider range (∼10 Hz–100 kHz). The lower parts of this frequency range, generally termed the extremely low frequency (ELF) range, along with still lower frequencies, have also been used in the study of thunderstorm-related ionosphere disturbances. In these cases the ELF radiated by the sprite or associated lightning has been examined. The interested reader is directed toward our earlier review (Rodger, 1999), recent works (e.g., Stanley et al., 2000; Cummer and Fullekrug, 2001), and papers contained in this special issue. It has been only slightly more than a decade or so since red sprites were 1rst reported in the literature. In this time a huge amount of experimental and theoretical work has been undertaken, leading to a better understanding of red sprites and the whole menagerie of new phenomena which have been identi1ed in the upper atmosphere. It has also lead to re-examination of many aspects of the physics and chemistry of thunderstorms, the middle and upper atmosphere, the ionosphere, and the magnetosphere. While the initial (primarily optical) experiments have appreciably advanced our understanding, much recent progress has come through the combination of techniques utilized by researchers with a wide variety of backgrounds and skills. The VLF techniques described in this article are part of this wider scheme, and will likely continue to be so. Acknowledgements The author would also like to thank Bernard Darnton of Dunedin for his support, and Neil R. Thomson of the University of Otago for helpful discussions.

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